Vehicle doors are tough, with all the door latch/lock/window crank mechanisms inside. Buckshot is a poor bet on a lot of auto sheet metal, I have seen several cases of failure to penetrate in real world shootings. Angle of incidence has a lot to do with it in many cases, round pellets impacting at an angle on rounded surfaces skid off easily without penetrating. Auto glass can be problematical in some cases too, again depending on range and angle of attack. Old police trick was to skip buckshot off the hood or trunk of a car to get the glass, taking advantage of this tendency.
That is why old shotgun hands keep slugs handy and confine buckshot to soft targets- in a pinch nothing beats the impact and penetration a slug can deliver. One more good lesson in knowing what your gun and your load will do in a given situation, and not counting on myths to deliver when what is needed is performance.
lpl/nc